Evaluation of an Alternate Method for Determining Yield ...

Evaluation of an Alternate Method for Determining Yield Strength Offset Values for Selective Laser Sintered Polymeric Materials

Chelsey Henry1, Keith Rupel2, Charles Park3, Joseph Costanzo3, Cary Kaczowka3, Kevin Malik4, and Sayata Ghose4 The Boeing Company 1Salt Lake City, UT 84116 2Berkeley, MO 63134 3Everett, WA 98204

4Tukwila, WA 98124

ABSTRACT

Due to the unique characteristics of Additively Manufactured (AM) polymeric materials, typical mechanical strength characterization methods such as those commonly used for traditionallyprocessed polymers or composite materials can produce results that do not accurately represent material capabilities. In order to characterize mechanical properties of these materials, new test and analysis methods are required. As part of the National Aeronautics and Space Administration (NASA) Advanced Composites Project (ACP), Boeing has evaluated true yield testing as an alternative or complimentary test to 0.2% offset yield testing for determining appropriate yield strength values of polymer materials. Previous testing has shown high strain, low modulus polymer materials such as selective laser sintered (SLS) Nylon 11 at elevated temperatures produce large variations in yield strength. The true yield test method was successful in finding the applied strain level when yield commences and appears to offer an increase in data robustness.

The material contained in this paper is based upon work supported by NASA under award No. NNL09AA00A through a sub award from the National Institute of Aerospace.

1. INTRODUCTION

As part of the Advanced Composites Project (ACP), NASA is partnering with the aerospace industry to significantly reduce the timeline to certify composite structure for commercial and military aeronautic vehicles. One of the objectives under this program is to develop cost and time efficient test methods to accurately evaluate mechanical properties (e.g., tensile, shear and compression strength) of Polyamide and Polyetherketoneketone materials that are fabricated with additive manufacturing (AM) processes. The effort described in the paper was a portion of the overall effort and initiated to investigate alternate yield strength determination methods and criteria for polymer materials. Specifically of interest was neat, thermoplastic additive manufactured materials and appropriate methods for their yield point determination. The standard yield point determination method currently employed for aerospace materials is colloquially referred to as the offset yield. The offset yield strength approach determines the material modulus in the linear or elastic portion of the curve and calculates the yield strength from where a specified offset parallel to the slope intercepts the stress-strain curve. While the ASTM standards (ASTM D638 and ASTM D695) do not define the exact strain levels used to calculate the linear modulus or the specified offset, industry standard practice is to use 0.1 and 0.3% strain, and 0.2% strain as the specified offset for determining yield strength. It is acknowledged that ASTM D638 does refer to other methods and offers corrections such as the toe-in correction and secant modulus method but the standard approach is to pre-program the 0.1 and 0.3% chord points and utilize the 0.2% offset. Using the industry standard values and practice in historical testing led to excessively conservative yield strength determinations in polymer materials at elevated temperature in particular. Nylon 11 tensile testing at temperature is an excellent example of this issue with Figure 1 being a typical historical tensile curve with 0.2% offset yield indicated.

Figure 1: Historical Nylon 11 SLS 165?F/Ambient Tension Test

The specific testing being employed in this paper is the true yield point test method; the intent of which is to determine the actual plastic deformation experienced by a test specimen after it is returned to a zero stress-state and permitted to relax. Previous testing and literature has demonstrated thermoplastics will exhibit a dimensional recovery stage that continues for a period of time after all stress has been removed. The true yield point of plastics test method was documented in a Datapoint Labs article [1] and references two earlier successful uses of this technique in application to polymethylmethacrylate, polystyrene and polycarbonate materials. The purpose of the true yield point test is to determine the location of the actual yield point on the stress-strain curve (Figure 2). The general idea of the procedure is to load the specimen, unload the specimen, measure the residual deformation after a fixed period of recovery time and then repeat the above steps at progressively higher loaded strain levels. The residual strains measured after each unloading and recovery time are then plotted as a function of the applied strains at each step (Figure 3). When the initial deformation exceeds the plastic limit of the material, a dramatic increase of residual strain is observed.

Figure 2: Stress strain curve showing the 3 different regions. The true yield point is circled [1]

Figure 3: Plot of residual extension versus initial applied extension [1] The residual strains after the initial few cycles are low and linearly increasing. These residual strains correspond to residual viscoelastic strains. For larger imposed extensions (around 4-5 mm),

the residual deformations become much larger, as plastic residual strains are also accumulated. To measure the true yield point from the curve, an onset or an intercept type construction may be applied. Of these, the onset construction presents a more conservative picture of this phenomenon. In this testing, it was decided to utilize onset of creep to define yield instead of the higher intercept of the linear and plastic portions of the response curve. It has been shown in the referenced Datapoint Labs article that this limit definition approach, and the subsequent yield strain, is independent of the time allowed for the material to recover.

2. EXPERIMENTATION

2.1 Material Definition and Specimen Fabrication

The Nylon 11 material used for testing was Rilsan D80 Natural ES from Advanced Laser Materials (ALM). Rilsan D80 was chosen due to a substantial amount of historical data present for this material. Standard ASTM D638 Type 1 tensile specimens were selected for this testing, with the modification that a label tab was extended from the side of the grip (Figure 4). This label tab has been shown to eliminate failures in the grip region associated with the commonly used labeling technique of engraving in the grip area directly. While this type of grip failure is not common in high elongation materials such as Nylon 11, it has been observed occasionally.

Figure 4: Tensile specimen geometry used

The test specimens were fabricated via the SLS process utilizing Vanguard equipment. Two forms of the specimens were printed, as-built net-shape specimen and rectangular blanks which were then machined using conventional methods to produce the ASTMD638 Type 1 specimen geometry. These two forms were used to evaluate the impact of surface roughness known to be associated with SLS processing on test results.

Relative to the equipment axis, a ZY-orientation (Figure 5) was selected for all test specimens, with a slight rotation around the Z-axis off of true ZY-orientation as a nod to SLS build considerations. Three builds were utilized to produce all the specimens. The first build consisted of rectangular blanks to be utilized in producing the machined specimens, and the second and third builds produced the as-built specimens. The specimens were located in a non-serial fashion in the build as part of a randomization effort to prevent an entire test grouping from coming from one particular build area and possibly influencing the results. Testing was also conducted in a random, non-sequential fashion in this test plan, resulting in further randomization.

Figure 5: Build orientation and 3D view of build

The machined specimens were machined with a standard ?" diameter end mill at 25 ipm and 12,000 rpm utilizing air as coolant. During testing of the machined specimens, particularly at 65?F, it was noticed the machined specimens were not of uniform thickness resulting in nonuniform load applications. At other temperatures, it appears the material was soft enough that nonuniform thickness was not a hindrance to load introduction and test results.

Specimen roughness was measured for both as-built and machined specimens. The as-built specimens had an Ra of approximately 700 micro-inches and the machined specimens had an Ra of 16-60 micro-inches.

2.2 Testing Procedure

Two moisture preconditioning levels were selected for this testing to be consistent with previous testing efforts.

1. Ambient ? 40 hours @ Room Temperature (RT) / 50% Room Humidity (RH) (ASTM D318 Procedure A and internal Boeing testing)

2. Wet ? 7 weeks @ 120?F / 95% RH

Specimen weight gain at the ambient and wet conditions was calculated by weighing extra specimens at the end of testing and then drying them in an oven at 215?F for 6 days. As-built and machined specimens exhibited similar moisture gain under both ambient and wet conditions. The wet conditioning resulted in 1.4% weight gain while the ambient conditioning resulted in 1.0% weight gain.

The true yield test method was evaluated in this work with three individual tensile test methods; a baseline tensile test, the multi-load tensile test and yield point check. The multi-load test is the heart of the true yield test method. The baseline tensile test results were intended to provide a reference data set. The yield point check test results were used to verify that the use of multiple loading cycles did not alter yield behavior relative to a single cycle of loading to the strain level being evaluated.

2.2.1 Baseline Tensile Test

The baseline tensile tests were conducted with both the as-built and machined configurations but with only a single replicate of each as it was not the focus of this effort given the availability of the historical data. Baseline specimens were loaded at typical ASTM D638 loading rates of 0.2 inches/min for all RT and above test conditions and 0.1 inches/min for the -65?F test condition with data collected at 10 Hz with an extensometer. Ambient conditioned specimens were temperature stabilized in the chamber for 10 minutes prior to test and wet specimens were temperature stabilized for 3 minutes prior to test to limit moisture loss at temperature.

2.2.2 Multi-load Tensile Test

The multi-load tensile tests were the primary data collection mechanism of this effort. The intent was to clearly indicate by sequentially loading and unloading the specimens to progressively higher strain levels where20 plastic deformation begins to occur and to what degree plastic deformation was occurring at higher strain levels. Loading rates, data collection rates and temperature stabilization holds matched those used for the baseline tests. The three minute thermal stabilization period of the wet specimens did lead to data interpretation issues and a greater temperature stabilization time is recommend in future testing with the multi-load method. The planned strain load stopping levels are shown in Table 1.

Table 1: Planned multi-load strain data collection points

Test Region Elastic

Transition

Cold 0.5%

0.75%

1.0% 1.25% 1.5% 1.75% 2.0% 2.25% 2.5%

Plastic

2.75%

3.0% 3.5%

RT 0.5%

1.0%

1.5% 2.0% 2.25% 2.5% 2.75% 3.0% 3.25% 3.5% 4% 5% 6.0% 7.0% 9.0% 11.0%

Hot/amb 0.25% 0.5% 0.75% 1.0% 1.5% 2.0% 3.0% 4.0% 5.0% 6.0% 7.0% 8.0% 9.0% 10.0% 11.0% 12.0%

15.0%

Hot/wet

1.0% 2.0% 4.0% 5.0% 6.0% 7.0% 8.0% 10.0% 12.0% 15.0%

The intent was to have several data points collected in each of the regions of the material response load curve; the elastic region, the transition region and the fully plastic region. In future testing, it is recommend greater emphasis be placed on the elastic region and the beginning of the transition region (below 4% applied strain) as data analysis has migrated to focus closely on the initiation of plastic deformation and not on the degree of plastic deformation at higher strain levels. Multi-load specimens were loaded to each strain level and then immediately ( ................
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